1. Field
The embodiments discussed herein are directed to an optical device and an optical transmitter.
2. Description of the Related Art
Recently speed enhancement advances have been made in optical communication technology, e.g., a transition from 10 Gb/s to 40 Gb/s. Development of an optical transmitter and an optical receiver, which are used in a communication system of 40 Gb/s or 100 Gb/s is anticipated.
Because a compact optical device in which an optical waveguide is used exhibits excellent characteristics, the optical device is commercially used in various devices such as an optical modulator, an optical receiver, and an optical switch. With progress of the communication system, a increased performance is demanded for the optical waveguide, and there is a need for improved optical waveguides.
For example, Japanese Patent Application Laid-Open No. 5-60929 and International Publication No. WO2002/023264 discloses an intersection optical waveguide. Japanese Patent Application Laid-Open No. 2007-94440 discloses an optical modulator in which a folding optical waveguide is used.
A degree of freedom of device design may be enhanced when a bending optical waveguide such as the folding optical waveguide intersects with another optical waveguide.
FIG. 1 illustrates a typical bending optical waveguide 110 with input 100 and output 120. FIG. 2 is a sectional view taken on a line 108-109′ of FIG. 1. The optical waveguide 110 of FIGS. 1 and 2 is a diffusion optical waveguide that is provided by diffusing Ti or the like in a substrate 130 made of LiNbO3 used in the optical modulator. In the substrate 130, a groove 140 may be formed outside a bending portion of the optical waveguide 110. An optical mode field 200 of the bending portion may be biased toward the outside of the bending portion as illustrated in FIG. 2. Accordingly, a medium such as the groove 140 (air layer having a refractive index n=1) having a refractive index difference larger than that of the optical waveguide 110 (LiNbO3 diffusion optical waveguide having a refractive index n=2.15) may be disposed outside the bending portion, which allows a low-loss bending optical waveguide to be implemented. The air layer may be an inert gas such as nitrogen or vacuum.
When another optical waveguide intersects with the bending portion of the optical waveguide 110, another optical waveguide intersects with the bending portion while the bending portion is returned to a linear optical waveguide, whereby an intersection portion may be formed by the methods disclosed in Japanese Patent Application Laid-Open No. 5-60929 and International Publication No. WO2002/023264. However, the device is lengthened, and a connection loss (up to 1 dB at two points) is generated in a connecting portion of the bending portion of the optical waveguide 110 and the linear optical waveguide.